Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
Image-guided radiotherapy and radiosurgery systems include gantry-based systems and robotic-based systems. In gantry-based systems, a radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. The radiation source may be rigidly attached to the gantry or attached by a gimbaled mechanism. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. Treatment angles are therefore limited by the rotation range of the radiation source and the degrees of freedom of a patient positioning system. In robotic-based systems, such as the CYBERKNIFE® Stereotactic Radiosurgery System manufactured by Accuray Incorporated of California, the radiation source is not constrained to a single plane of rotation and has five or more degrees of freedom.
In conventional image-guided radiation treatment systems, patient tracking during treatment is accomplished by comparing two-dimensional (2D) in-treatment x-ray images of the patient to 2D digitally reconstructed radiographs (DRRs) derived from the three dimensional (3D) pre-treatment imaging data that is used for diagnosis and treatment planning. The pre-treatment imaging data may be computed tomography (CT) data, magnetic resonance imaging (MRI) data, positron emission tomography (PET) data or 3D rotational angiography (3DRA), for example. Typically, the in-treatment x-ray imaging system is stereoscopic, producing images of the patient from two or more different points of view (e.g., orthogonal).
A DRR is a synthetic x-ray image generated by casting (mathematically projecting) rays through the 3D imaging data, simulating the geometry of the in-treatment x-ray imaging system. The resulting DRR then has the same scale and point of view as the in-treatment x-ray imaging system, and can be compared with the in-treatment x-ray imaging system to determine the location of the patient. To generate a DRR, the 3D imaging data is divided into voxels (volume elements) and each voxel is assigned an attenuation (loss) value derived from the 3D imaging data. The relative intensity of each pixel in a DRR is then the summation of the voxel losses for each ray projected through the 3D image. Different patient poses are simulated by performing 3D transformations (rotations and translations) on the 3D imaging data before the DRR is generated.
In some image-guided systems, the 3D transformations and DRR generation are performed iteratively in real time, during treatment. In other systems, such as the CYBERKNIFE® Stereotactic Radiosurgery System manufactured by Accuray Incorporated of Sunnyvale, Calif., a set of DRRs (in each projection) corresponding to an expected range of patient poses may be pre-computed before treatment begins.
Each comparison of an in-treatment x-ray image with a DRR produces a similarity measure or, equivalently, a difference measure (e.g., cross correlation, entropy, mutual information, gradient correlation, pattern intensity, gradient difference, image intensity gradients) that can be used to search for a 3D transformation that produces a DRR with a higher similarity measure to the in-treatment x-ray image (or to search directly for a pre-computed DRR as described above). When the similarity measure is sufficiently maximized (or equivalently, a difference measure is minimized), the 3D transformation corresponding to the DRR can be used to align the 3D coordinate system of the treatment plan with the 3D coordinate system of the treatment delivery system, to conform the relative positions of the radiation source and the patient to the treatment plan. In the case of pre-computed DRRs, the maximum similarity measure may be used to compute a differential 3D transformation between the two closest DRRs.
Image-guided radiation treatment systems provide an effective and non-invasive solution to the treatment of a wide variety of pathological anatomies (pathologies). However, certain types of pathologies present a particularly difficult treatment challenge. These types of pathologies may include relatively small tumors in relatively large organs such as the lungs, liver and pancreas, where the density of the tumor is very close to the density of the surrounding healthy tissue and the tumor is difficult to visualize using standard imaging technologies (e.g., x-ray imaging). Typically, these tumors are approximately 15 millimeters or less in diameter, but larger tumors may present the same or similar problems depending on the type of tumor and the specific organ. The challenge is particularly difficult when the tumor is in motion due to patient breathing during treatment, and the tumor must be tracked in real time or near real time.
One conventional method of dealing with the motion of a target region during radiation treatment involves the image tracking of fiducial markers that are placed in or near the target region. The position and motion of the fiducial markers is correlated with the position and motion of the target region so that real-time correction of the position of the treatment beam to follow the motion of the target region may be realized. This approach has the disadvantage of requiring an invasive surgical procedure to place the fiducial markers.
Conventional image-guided treatment systems attempt to locate pathologies using DRRs and in-treatment x-ray images with relatively large fields of view in an attempt to maximize image information. However, in the case of the small, poorly differentiated and moving pathologies discussed above, the conventional approach may be computationally expensive and time-consuming, slowing the imaging processing functions of the treatment system and rendering the output data rate too low for accurate tumor tracking.